1. Field of the Invention
The present invention relates to a device and a method for image pickup control for driving an image sensor to pick up a scene by, e.g., long exposure.
2. Description of the Background Art
It is a common practice with a camera of the type using a silver halide photosensitive type of film to shoot a night view, night sky or similar dark scene with low brightness by selecting a low shutter speed for long exposure or by using a high-speed film. This is also true with a digital still camera using a CCD (Charge Coupled Device) image sensor. Specifically, to shoot a dark scene, a digital still camera increases the amplification ratio and gain of image signals to thereby vary sensitivity.
A low shutter speed, however, increases the period of time over which photodiodes arranged in the CCD image sensor store signal charges, thereby causing dark current to excessively accumulate. Further, when the gain of an image signal is increased, a noise level rises together with the level of necessary pixel signals and therefore lowers the S/N (Signal-to-Noise) ratio of the image signal.
Japanese patent laid-open publication No. 9-168118, for example, discloses a solid-state image pickup device constructed to reduce dark current in a CCD chip. The image pickup device disclosed includes pulse generating means for generating charge reading pulses, horizontal drive pulses, vertical drive pulses and reset pulses, a plurality of drive means, and control means. The control means controls the feed of the each of the drive pulses and reset pulses to particular one of the drive means in correspondence to the feed of the charge reading pulses.
In the image pickup device taught in the above document, when photodiodes are caused to store charges over an exposure time of about 1/30 second, the feed of the vertical drive pulses, horizontal drive pules and reset pulses is interrupted in order to reduce heat to be generated and therefore dark current. When the exposure time is as short as about 1/60 second, the device is driven in a usual manner with the above pulses being continuously fed.
Japanese patent laid-open publication No. 8-306908 proposes a charge detecting element including a first source-follower circuit made up of transistors Tr1 and Tr2 and a second source-follower circuit made up of transistors Tr3 and Tr4 and following the first source-follower circuit. Different voltages V1 and V2 are respectively applied to the load transistors Tr2 and Tr4 in order to reduce the gate capacitance of the first source-follower circuit while improving the frequency characteristic of the second source-follower circuit.
As stated above, it has been customary to reduce noise to appear in charge transfer paths and a charge detecting element over a relatively short period of time by interrupting the feed of pulses to a drive system. However, a problem with the previously mentioned laid-open publication No. 9-168118 is that during exposure unnecessary charges cannot be discharged from the charge transfer paths because charge transfer is interrupted. Moreover, the amount of unnecessary charges increases with an increase in exposure time. The above document gives no consideration to the discharge of charges accumulating during long exposure. Therefore, the charge transfer interruption scheme cannot simply be used to shoot a night view or a starry sky over an exposure time as long as several seconds to several ten seconds. Such a long exposure time gives an amount of exposure of 5 EV (Exposure Value) or above that is far greater than an amount assigned to a daytime shot.
Assume that exposure is effected over a long period of time without using the above-described conventional technology. Then, the charge detecting element built in an image sensor is driven over a long period of time and therefore generates more heat, heating elements around it. In this condition, photodiodes adjoining the charge detecting element generate unnecessary charges ascribable to heat more than the other photodiodes, resulting in an irregular distribution unnecessary charges. Presumably, the generation of unnecessary charges during long exposure is dependent on the geometrical distance between the individual photodiode and the charge detecting element in the photosensitive array and the structure of the image sensor. The temperature elevation of the photodiodes directly translates into an increase in dark current, generating charges having no relation to the amount of incident light.
For example, assume that a night view, a fine starry sky or similar scene whose major portion has a low brightness level is shot over an exposure time of more than 3 seconds. Then, fog appears noticeably in part of the resulting image around the charge detecting element. More specifically, the pixel level increases in accordance with the exposure time to such a degree that the above-mentioned part of the image appears somewhat white or appears in an unexpected color. The photographic fog is aggravated as the exposure time further increases. Such fog appears not only in an image shot at night, but also in an image shot by long exposure using a light attenuating filter or a small lens opening. An exclusive camera for shooting night scenes and starry skies usually includes a Peltier device that forcibly cools off an image sensor to thereby reduce the temperature elevation of the image sensor and therefore noise to appear in images. This kind of cooling device, however, is not practical when it comes to a digital camera that is handy, common equipment.
Local fog described above occurs without regard to an exposure mode, i.e., an automatic exposure mode or a manual exposure mode. To attain high image quality, it is a common practice to limit the exposure time during which charges are generated in an image sensor. This, however, makes it impossible to implement a camera suitable for nighttime shots as well as for daytime shots. Moreover, it is difficult to remove unnecessary charges ascribable to long exposure or to remove the influence of the unnecessary charges from pixel signals read out of an image sensor because the unnecessary charges are locally generated. In this manner, conventional technologies cannot easily cope with the local increase of dark current, i.e., an irregular dark current distribution ascribable to long exposure, i.e., heat generated by the charge detecting element.